How long does a deck last

A pressure-treated pine deck lasts 15 to 25 years. Cedar lasts 15 to 20. Composite decking lasts 25 to 30, on a frame that is usually still pressure-treated pine and therefore still ages at pine's rate.

That last point is the one worth holding onto. What ends a deck is almost never the boards you walk on. It is the frame, the posts and the ledger, which nobody looks at.

What lasts how long

The five things that actually decide it

Flashing over the ledger. A deck with proper flashing where it meets the house lasts twice as long as one without, and the difference is a $30 piece of metal installed on day one. Unflashed ledgers rot, and they take the house's rim joist with them.

Whether the posts touch the ground. A post set in soil, or in concrete with no drainage, wicks water and rots at grade. A post on a galvanized bracket above a concrete pier stays dry and outlasts the deck.

Drainage between boards. Boards fitted tight, or with debris packed in the gaps, hold water against the joists. The joist tops are where a frame rots first, and joist tape at $0.50 to $1.50 a linear foot during construction adds years.

The fasteners. Modern pressure-treated lumber is corrosive to ordinary galvanized steel. Hot-dip galvanized or stainless is required; electro-galvanized fails. A deck whose fasteners are corroding is failing at every connection at once.

Finish, or the decision not to. A stained deck lasts longer than a bare one. A deck finished once in 2009 and never again is worse off than one never finished at all, because a failed film traps moisture.

Reading your own deck

Twenty minutes each spring:

• Screwdriver test the ledger, the rim joist behind it, joist ends, post bases and stair stringers. Soft is rot.
• Look for the flashing over the ledger. Absence is the most common finding.
• Shake the railing at the top of the stairs. Movement is a post attachment problem.
• Bounce on it. A deck that moves has a framing problem.
• Look at the fasteners for rust, backing out, or hangers fastened with the wrong nails.
• Check the boards for cupping, splitting, and soft spots around fasteners.

The frame findings are the ones that matter. Boards can be replaced; a rotten ledger is a structural repair and a collapse risk.

Extending it

• Clean and reseal on cycle: every two to four years for semi-transparent stain
• Keep debris out of the gaps, especially where a deck sits under trees
• Keep planters off the boards, or on feet, because a planter sitting flat holds water permanently
• Clear snow with a plastic shovel, and keep de-icing salt off wood
• Trim back planting so air moves under and around it
• Replace corroding fasteners before the connections loosen

A deck maintained this way reaches the top of its range. One left alone reaches the bottom.

Frequently asked questions

Can I put composite decking on an old frame?

Only if the frame is sound and the joist spacing suits it: most composite needs joists at 16 inches on center, some at 12 for diagonal patterns. Adding joists is $100 to $300 each, and putting 30-year decking on a 20-year frame is a decision worth making deliberately.

Does composite decking really last longer?

The boards do, and they do not rot, splinter or need staining. They fade, they scratch, they get very hot in direct sun, and the early generations stained badly. The frame underneath is still wood, which is why a composite deck is not a maintenance-free deck.

How do I know if my deck is safe?

The ledger, the posts and the railing, in that order. If the ledger is unflashed or soft, or the railing moves when you push it, those are the two that fail with people on the deck. An inspection by a contractor is $150 to $400.

When should I replace rather than repair?

When the ledger or the rim joist behind it is rotten, when more than a quarter of the joists are soft, when the posts have failed at grade, or when the deck does not meet current code for railing height and stair geometry and you are opening it up anyway.

Is a permit needed for a rebuild?

For structural work, yes in almost every jurisdiction. The footing and framing inspections are the only independent check anyone will make on the parts that carry people.